1. Field of the Invention
This invention is directed to a method for providing support and stabilization to an underground mine cavity which has been penetrated by at least one drilled hole by the use of foam to pressurize the underground mine cavity while it is being hydraulically mined.
2. Description of the Prior Art
One of the greatest concerns in hydraulic borehole mining is the possibility of a catastrophic cavity collapse during the mining process. This is always a pending danger when, as in existing procedures, the pressure in the cavity is reduced to essentially atmospheric conditions. When the reduction to atmospheric pressure occurs, the integrity of the cavity is maintained primarily by the beam strength of the ceiling and the shear strength of the walls. In order to avoid or minimize the possibility of a cavity collapse, it is beneficial and sometimes necessary to maintain a positive pressure within the cavity.
Underground support has been provided by inflatable flexible containers of various shapes and sizes. A good explanation of some of these prior art containers can be found in U.S. Pat. No. 2,990,166 to M. A. Walsh. As shown in the Walsh patent, proper support is provided when the inflated containers span the roof to floor and come into bearing contact with them (column 4, lines 5-14). One disadvantage with these types of supports is that they provide immediate support only to the mine bearing surfaces in contact therewith and perhaps a small adjacent roof area. Thus, in auger mining where the boreholes may easily be 100 or more feet in length and around 18 to 60 inches in diameter an exceedingly large, and expensive inflatable support would have to be provided to fill the borehole cavity. Obviously, the presence of an inflatable container within an underground cavity would preclude use of a hydraulic borehole mining tool since contact could not be made by the fluid cutting jet with the materials to be mined.
Another method that has been used in the past to support the overburden, especially in tunnelling, is referred to as the plenum process or compressed-air method. A book entitled "Practical Tunnel Driving," by Harold Richardson and Robert S. Mayo--1975 McGraw Hill Book Co. (1st Edition 1941) on pages 275-300 describe this method. Another description is found in the book "Tunnels and Tunnelling" edited by C. A. Pequignot, Hutchinson and Co., Ltd., London (1963), pages 158 to 184. Essentially, this method provides compressed air to a tunnel lining which has been locked off from the free air side to allow the working face area to be under increased air pressure. Depending on such variables as the ambient materials making up the tunnel, the depth below water level, and the safety considerations for the workers, the pressure and amount of compressed air is determined and controlled to prevent the water and surrounding material from collapsing on the workers or from exploding outwardly towards the surface of the water.
U.S. Pat. No. 4,072,015 to R. J. Morrell discloses a method and apparatus to provide temporary ground support in a mine. After an auger or other type of hole has been bored into an underground mine, at least one inflatable bladder with a fluid conduit therethrough is inserted into the hole. Once the bladder is inflated it extends to fill the diameter of the hole and to act as a plug. Thereafter, fluid is forced through the fluid conduit past the bladder and into the hole cavity to act in conjunction with the bladder as a ground support for the hole. Appropriate fluids, including foams, valves, meters, and conduits are placed at the input to the fluid conduit to control and measure the fluid being forced into the bladder and the hole cavity. Morrell's patent differs from applicant's invention because insertion of a bladder into an underground mine cavity would preclude the mining of the hydrocarbonaceous materials as practiced by applicant.
Richardson et al., U.S. Pat. No. 4,232,741, have used self-foaming aqueous solutions to temporarily plug subterranean reservoirs. This was accomplished by injecting an aqueous liquid solution which contains nitrogen gas-generating reactants, a foaming surfactant and a pH controlling system arranged so that the solution remains relatively unreactive within the well but forms a relatively immobile foam within the pores and other openings within the reservoir formation. Richardson's et al patent differs from applicant's invention in that the foam generated is immobile and the foam is utilized in a subterranean formation into which a well is completed. Applicant's invention utilizes a mobile foam floating on water within a subterranean reservoir without a well.
Until now no one has disclosed a method for generating a mobile foam within the reservoir during the borehole mining of minerals which foam generates sufficient pressure to serve as a overburden support system.